The present invention relates in general to an exhaust gas purifying system used with an internal combustion engine of a motor vehicle and more particularly to an electronically controlled two barrel carburetor for the internal combustion engine having at the exhaust system thereof a triple catalytic converter, the electronically controlled two barrel carburetor being operated in response to the composition of the engine exhaust gases being fed into the triple catalytic converter.
It is recognized that a triple catalytic converter arranged in an exhaust system of an internal combustion engine can convert the harmful compounds such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) contained in the engine exhaust gases into harmless compounds by the activating action of a catalyst or catalysts contained in the triple catalytic converter. In this kind of catalytic converter, the highest purifying performance of it occurs when the air-fuel mixture subjected to conventional combustion in the combustion chambers of the engine is kept within an extremely limited range of the stoichiometric air-fuel ratio.
A variety of devices have thus far been proposed and put into practice to maintain the air-fuel mixture supplied into the combustion chambers for its conventional combustion at the stoichiometric ratio, one of which is an electronically controlled carburetor which can electronically control the amount of fuel and/or air, accordingly the air-fuel ratio of the air-fuel mixture, being fed into the intake manifold of the internal combustion engine is in accordance with the composition of the exhaust gases from the engine. Thus, the air-fuel mixture being fed into the combustion chamber for conventional combustion is kept at the stoichiometric air-fuel ratio. This type of carburetor has been proposed because of an obvious mutual relation existing between the constituents of the exhaust gases from the engine and the air-fuel ratio of the mixture fed into the combustion chambers for the conventional combustion, as is well known to those skilled in the art.
However, in this kind of a carburetor, the air-fuel ratio of the air-fuel mixture being fed into the intake manifold is varied in steps by the valve operation of an electric controller used for controlling the amount of fuel or air feeding into the intake manifold. Furthermore, when the controller is an ON-OFF operation type solenoid valve, a remarkable variation of the air-fuel ratio of the mixture occurs in the intake manifold at the moment when one state of the solenoid valve is changed to the other, thereby causing the air-fuel ratio of the mixture supplied the combustion chambers to markedly deviate from the stoichiometric ratio. Furthermore, in a worst case, "hunting phenomenon" occurs.